Fuel gas streams obtained from various sources usually contain, in addition to methane and other combustible hydrocarbons, substantial quantities of carbon dioxide and nitrogen. A typical mixed gas stream, such as that obtained from enhanced oil recovery (EOR) operation may contain in addition to methane, in the order of about 50% or more of non-combustible gases (such as N.sub.2 and CO.sub.2). To recover the methane in purified form from such gas mixtures the conventional processes employ a chemical solvent, such as monoethanolamine, to separate accompanying components from the desired methane. In typical prior art operation, for example, the crude off-gas is subjected to cryogenic distillation, whereby most of the C.sub.2 + hydrocarbons and part of the carbon dioxide are separated. The remaining gas fraction is treated with the chemical solvent to extract the bulk of the remaining CO.sub.2 and the separated crude methane product is subjected to drying and refrigeration to remove water and residual CO.sub.2. The nitrogen remaining is removed from the resulting product in a Nitrogen Rejection Unit (NRU), such as that described in U.S. Pat. Nos. 4,411,677 and 4,504,295 and in a paper by Alvarez, M. R., et al., published in Oil and Gas Journal, Aug. 20, 1984 at pages 95-99.
In these conventional processes employing chemical solvent for CO.sub.2 removal from fuel gas, the gas driers and the refrigeration systems required represent major capital and operating expenses. Moreover, the use of such chemical solvent systems to remove the carbon dioxide is an energy intensive process requiring steam to regenerate the solvent.
The separation of methane from gas mixtures containing also carbon dioxide by PSA operation, is disclosed in prior art patents. Among these, for example, U.S. Pat. No. 4,077,779 discloses a PSA system generally applicable to bulk separation of various gas mixtures, including the separation of CO.sub.2 from its admixture with methane, in a six step cycle, wherein following selective adsorption of one of the components of the mixture, the adsorbent bed is rinsed with part of the adsorbed component at superatmospheric pressure. The pressure in the rinsed bed is lowered to an intermediate level to desorb the same, the withdrawn gas in this step being employed in the high pressure rinse step. The bed is next purged with an extraneous gas (air or inert) and then evacuated to remove the purge gas, following which the bed is brought back to superatmospheric pressure level for repetition of the cycle.
Among the objects of the present invention is to provide a simpler and less costly system and process than that conventionally employed for recovery of methane from mixed gas streams containing the same.